FACULTY OF ENGINEERING
LAB SHEET
ETN 2016
ANALOG COMMUNICATIONS
TRIMESTER 3 (2014/2015)
Experiment 1:
TM201 – Amplitude Modulation
Note: Students are advised to read through this lab sheet before doing experiment. On-thespot evaluation will be carried out during or at the end of the experiment. Your performance,
teamwork effort, and learning attitude will count towards the marks.
1. Objectives
Experiment Part 1: Amplitude Modulation (AM)
 To examine a message signal, a carrier signal and an AM modulated waveform in
time domain.
 To measure the modulation index of an AM signal.
 To examine a message signal, a carrier signal and an AM modulated waveform in
frequency domain.
Experiment Part 2: AM Demodulation
 To demonstrate the demodulation of AM signal.
2. Introduction
a) Modulation
To communicate over long distance we can send a radio frequency (RF) signal between
two antennas, one at the transmitting end and the other at the receiver. Frequencies used
by AM transmissions are typically between 200kHz and 25MHz. A typical radio
frequency of say, 1MHz is much higher than the audio frequencies present in the human
voice.
We appear to have two incompatible requirements. A radio system uses frequencies like
1MHz to transmit over long distance, but we wish to send typical voice frequencies of
between 300Hz and 3.4kHz which are quite impossible to transmit by radio signals. This
problem can be overcome by using a process called “modulation”.
A radio system can easily send high frequency signals between a transmitter and a
receiver but this, on its own, conveys no information. Now, if we were to alter the high
frequency signals (in amplitude or frequency or phase or any combination of them) in
correspondence with the variation of the amplitude of the message signal, we could use it
to send information.
Modulation is a process where a “carrier wave” is systematically altered (in amplitude or
frequency or phase or any combination of them) in correspondence with the variation of a
modulating signal, the message signal. The resulting modulated signal will carry the
message information in its amplitude or frequency or phase or any combination of them.
YL Foo, 2014 Feb
2
b) Amplitude Modulation (AM)
In Amplitude Modulation, the carrier signal sc (t )  Ac cos c t is modulated in amplitude
s AM (t )  Ac 1  m(t ) cosc t
by m(t) to produce:
Figure 1: AM
Ac
Carrier
10
8
6
4
2
0
-2
-4
-6
-8
0
20
40
60
80
100
120
Message signal or modulating waveform
20
15
10
Ac
5
0
-5
-10
-15
-20
0
20
40
60
80
100
120
Modulated AM waveform
c) Depth of modulation
Let us consider the following signals:
sc (t )  Ac cos c t
Carrier:
m(t )  m cos m t
Modulating signal:
Modulated signal:
s AM (t )  Ac 1  m cosmt  cosc t
The amount by which the amplitude of the carrier wave increases or decreases depends
on the amplitude of the information signal and is called the modulation index.
A  Ac Amax  Amin
A  Amin
A B
Modulation index, m  max

 max

Ac
2 Ac
Amax  Amin A  B
Percentage of modulation  m  100%
YL Foo, 2014 Feb
3
i) Undermodulation, m < 1
Figure 2: m < 1
A
1.5
1
B
0.5
Ac(mi
n)
Ac(ma
Ac
x)
0
-0.5
-1
-1.5
0
5
10
15
20
25
30
35
40
45
Under this modulation condition, a simple noncoherent demodulation technique such
as envelope detector can be used to recover the original message signal without
distortion.
ii) Overmodulation, m > 1
Figure 3: m > 1
2.5
2
1.5
1
0.5
0
-0.5
-1
-1.5
-2
-2.5
0
5
10
15
20
25
30
35
40
45
Under this modulation condition, noncoherent demodulation will not be able to
recover the original message signal without distortion.
When the message signal is constantly changing, as in a voice or music signal, the
frequency of the modulating envelope and the percentage of modulation are
constantly changing. This constant change makes it practically impossible to
determine the modulation index from the waveform displayed on an oscilloscope.
A solution to the above problem is to set the oscilloscope display to X-Y mode. In
this method, the X input is Channel 1 (the message signal) and the Y input is Channel
2 (the modulated signal).
YL Foo, 2014 Feb
4
A
B
Figure 4 : XY mode oscilloscope display
The modulation index is given by: m 
A B
A B
d) Frequency spectrum
Figure 5 : amplitude of spectral components of AM signal (1-sided plot)
m2Ac
m1Ac m3Ac
AM
modulation
lower
sideband
carrier
Ac
m2Ac/2
m3Ac/2 m1Ac/2
upper
sideband
m2Ac/2
m1Ac/2 m3Ac/2
f
fc-f2
fc+f2
fc
fc-f1
fc+f1
fc-f3
fc+f3
The amplitude spectrum of the following signal m(t)
0
f1
f2 f3

m(t )  Ac m1 cos1t  m2 cos2 t  m3 cos3t
and the corresponding AM signal are shown in Figure 5.

Note that each modulating frequency component produces its own upper and lower side
frequencies around the carrier frequency. All the upper side frequencies are grouped
together and referred to as the upper sideband (USB) and all the lower side frequencies
form the lower sideband (LSB).
If the frequency range of the message signal is from 0 to f3, then, the bandwidth of the
message signal, BW m ( t )  f 3 . The corresponding AM modulated signal will occupy a
frequency range from f c  f 3 to f c  f 3 , and the bandwidth of the corresponding AM
modulated signal is given by:
BW AM   f c  f 3    f c  f 3   2 f 3  2 BW m (t ) .
YL Foo, 2014 Feb
5
3. Glossary















Amplitude modulation (AM) – the process of combining a message signal with a
carrier signal that causes the message signal to vary the amplitude of the carrier signal
AM signal – an amplitude modulated signal that contains carrier signal and two
sidebands : the lower sideband and the upper sideband
Audio frequency (AF) – frequencies that a person can hear, typically ranging from
20Hz to 20kHz
Balanced modulator – an amplitude modulator that can be adjusted to control the
amount of modulation
Bandwidth – the frequency range, in hertz, between an upper and a lower frequency
limits
Carrier signal – a single, high-frequency signal that can be modulated by a message
signal and transmitted
Demodulation – the process of recovering or detecting an intelligent/ message signal
from a modulated carrier signal
Double sideband (DSB) – an amplitude modulated signal in which the carrier is
suppressed, leaving only the two sidebands : the lower sideband and the upper
sideband
Envelopes – the waveform of the amplitude variations of a high frequency signal
Harmonics – signals with frequencies that are an integral multiple of the fundamental
frequency
Message signal – any signal that contains information
Mixer – an electronic circuit that combines two frequencies
Modulation – the process of combining a message signal with a carrier signal that
causes the message signal to vary a characteristic (such as amplitude, frequency,
phase, etc) of the carrier signal
Radio frequency (RF) – the transmission frequency of electromagnetic signals. RF
frequencies range from about 300kHz to 1GHz range
Sidebands – the frequency bands on each side of the carrier frequency that are formed
during modulation; the sideband frequencies contain the intelligence of the message
signal
4. Material & Equipment Required
i)
ii)
iii)
iv)
v)
vi)
ANACOM 1/1, ANACOM 1/2
Power Supply : +12V at 1A, -12V at 1A (LJ PS2 IC Power 60 or PS4 System Power
90)
Set of 4mm patching leads
Dual Trace Oscilloscope
Function Generator
Spectrum Analyzer
YL Foo, 2014 Feb
6
5. Precautions
i)
ii)
iii)
All ground terminals at the power supply must be shorted.
The negative terminal of the oscilloscope probe must be connected to the 0V
reference on the circuit board during all measurements.
Remember to switch off the power supply at the end of each subsection, this is to
reduce interference to other experiment.
6. References
i)
ii)
iii)
LJ Technical System, “An Introduction to Analog Communications – Curriculum
Manual AT02”
LJ Technical System, “An Introduction to Analog Communications – Student
Workbook AT02”
Taub and Schilling, “Principles of Communication Systems”, McGraw-Hill, 1986,
Chapter 3.
YL Foo, 2014 Feb
7
YL Foo, 2014 Feb
9
7. Experiment Procedure
7.1 Experiment Part 1: Amplitude Modulation (AM)
7.1.1. Message Signal, Carrier Signal, and AM Signal
i)
Connect the ANACOM 1/1 board to the power supply as shown below:
Figure 6 : Power supply connection
+12V –12V +5V
Power Supply PU2
GND
+12V
0V
-12V
ANACOM 1/1
Board
ii)
iii)
iv)
v)
vi)
vii)
ANACOM 1/1 board set-up
 AUDIO INPUT SELECT is switched to INT position.
 MODE is switched to DSB.
 SPEAKER is switched to OFF.
 In the AUDIO OSCILLATOR both the AMPLITUDE PRESET and the
FREQUENCY PRESET should be set to maximum (fully clockwise).
 In the BALANCED MODULATOR & BANDPASS FILTER CIRCUIT 1, the
BALANCE PRESET should be set to maximum (fully clockwise).
 In the OUTPUT AMPLIFIER, decrease the gain to its minimum value (fully
counter clockwise).
Switch on the power supply.
Connect the oscilloscope Channel 1 probe to tp1 (test point 1), the message signal
input of the MODULATOR and trigger on Channel 1. Set timebase to 50s/div. This
is an audio signal which is going to be used to amplitude modulate a carrier signal.
Determine the frequency and peak-to-peak voltage of the audio signal from the
display.
Set the oscilloscope to DUAL display mode. Connect the oscilloscope Channel 2
probe to tp9, the carrier signal input of the MODULATOR and trigger on Channel 2.
Set the Channel Coupling to AC. Set the oscilloscope timebase to 0.2s/div. This is a
high frequency signal that is going to be used to carry the message signal. Adjust the
vertical scale if necessary.
Determine the frequency and peak-to-peak voltage of the carrier from the display.
Connect the Channel 2 probe to tp3, the modulated AM signal at the output of the
MODULATOR. Set the oscilloscope vertical mode to DUAL, trigger on Channel 1
(the message signal), and timebase to 50s. Adjust the vertical scale if necessary.
Relate the two waveforms displayed.
Adjust the AMPLITUDE PRESET and the FREQUENCY PRESET in the AUDIO
OSCILLATOR.
Observe if the envelope of the AM signal (Channel 2) changes in correspondence to
the changes in the amplitude and frequency of the message signal (Channel 1).
Display three types of waveforms: under-modulation, 100% modulation, and overmodulation.
7.1.2 Modulation Index and Percentage of Modulation
i)
Continue with the previous setting. Ensure that in the AUDIO OSCILLATOR, both
the AMPLITUDE PRESET and the FREQUENCY PRESET are set to maximum
(fully clockwise).
ii)
Connect oscilloscope Channel 1 probe to tp1, the message signal. Set the oscilloscope
to trigger on Channel 1.
iii)
Connect Channel 2 probe to tp3, the modulated AM signal.
Measure the vertical height, A and B (in Volts) from the peaks and valleys
respectively. Calculate the modulation index and the percentage of modulation.
iv)
Set the oscilloscope to XY mode. (for GOS-652 oscilloscope: set VERT mode to XY, set TRIGGER source to X-Y, and set timebase to X-Y)
Measure A and B, and calculate the modulation index. Compare your results to the
one obtained in the previous step.
7.1.3 Frequency Spectrum
i)
Connect the ANACOM 1/1 board to the power supply.
ii)
ANACOM 1/1 board set-up
 AUDIO INPUT SELECT is switched to EXT position.
 MODE is switched to DSB.
 SPEAKER is switched to OFF.
 In the BALANCED MODULATOR & BANDPASS FILTER CIRCUIT 1, the
BALANCE PRESET should be set to maximum (fully clockwise).
 In the OUTPUT AMPLIFIER, decrease the gain to its minimum value (fully
counter clockwise).
iii)
Connect the output of a Function Generator (positive terminal) to the EXTERNAL
AUDIO INPUT, tp16 of the ANACOM 1/1 board. The ground terminal of the
Function Generator must be connected to the 0V reference of the ANACOM 1/1
board.
iv)
Power on the Function Generator and setup as follows :
 Sine wave
 10 kHz
 2 V peak-to-peak
v)
Switch on the power supply of ANACOM 1/1.
vi)
Power on and set the Spectrum Analyzer as follows:
 FREQ, CENTER : 10kHz
 SPAN, WIDTHSPAN : 10kHz
Monitor the message signal at tp1 on the spectrum analyzer. Use the MKR key to
activate the marker and use Scroll Key to move the marker around. The frequency
YL Foo, 2014 Feb
11
vii)
viii)
and amplitude level corresponding to the marker location are displayed on the upper
right hand corner of the display screen.
Examine the spectrum (frequency in kHz & amplitude in Volt) of the message signal.
Set the spectrum analyzer as follows:
 FREQ, CENTER : 1MHz
 SPAN, WIDTHSPAN : 40kHz
 Other settings are the same as before.
Monitor the carrier signal at tp9 on the spectrum analyzer.
Examine the spectrum (frequency in kHz & amplitude in Volt) of the carrier signal.
Monitor the transmitted AM signal at tp3 on the spectrum analyzer.
Examine the spectrum (frequency in kHz & amplitude in Volt) of the AM modulated
signal.
Assess Your Comprehension:
1. Describe the characteristics of a message signal, a carrier signal and an AM modulated
signal.
2. Demonstrate your understanding on modulation with reference to AM modulation that
you have experienced in this experiment.
3. Compare the amplitude spectrums obtained from measurements in 7.1.3 with the
theoretical amplitude spectrums. Comment on the differences.
4. Discuss the main function(s) of the RF amplifier stage.
5. Consider a message signal with frequency content ranging from 0Hz to 10kHz.
Determine the bandwidth of the message signal and the corresponding AM modulated
signal.
YL Foo, 2014 Feb
12
7.2 Experiment Part 2: AM Demodulation
7.2.1 Non-coherent AM demodulation
i)
In this session, we directly connect the transmitter output to the receiver input instead
of using the pair of antennas. Connect the ANACOM 1/1 and ANACOM 1/2 boards
to the power supply as shown below:
Figure 7 : Power supply connection
+12V –12V +5V
Power Supply
GND
+12V
0V
-12V
ANACOM 1/1 Board
ii)
iii)
iv)
v)
vi)
+12V
0V
ANACOM 1/2 Board
ANACOM 1/1 Board set-up
 AUDIO INPUT SELECT is switched to INT position
 MODE is switched to DSB
 SPEAKER is switched to OFF
 In the AUDIO OSCILLATOR both the AMPLITUDE PRESET and the
FREQUENCY PRESET should be set to maximum (fully clockwise).
 In the BALANCED MODULATOR & BANDPASS FILTER CIRCUIT 1, the
BALANCE PRESET should be set to maximum (fully clockwise).
 In the OUTPUT AMPLIFIER, increase the gain to its maximum value (fully
clockwise)
 The TX OUTPUT SELECT of ANACOM 1/1 should be set to ANT.
 Use jumper wire to connect the TX OUTPUT socket of ANACOM 1/1 to the RX
INPUT socket of ANACOM 1/2.
ANACOM 1/2 Board set-up
 In the AUDIO AMPLIFIER, switch the SPEAKER to ON and decrease the
VOLUME preset to its minimum value (fully counter-clockwise)
 The RX INPUT SELECT of ANACOM 1/2 is switched to ANT.
 In the RF AMPLIFIER, switch the TUNED CIRCUIT SELECT to INT (internal)
position and increase the RF AMPLIFIER GAIN CONTROL to maximum (fully
clockwise).
 Set the AGC switch to the IN position.
 Set the DETECTOR switch to the DIODE position.
 Switch the BEAT FREQUENCY OSCILLATOR to the OFF position.
Clip both ends of a wire to the antennas of both ANACOM 1/1 and ANACOM 1/2.
Switch on the power supply.
Increase AUDIO AMPLIFIER’s VOLUME preset until sound is audible from the
speaker.
YL Foo, 2014 Feb
13
vii)
viii)
ix)
x)
xi)
xii)
Adjust the Tuning Control on ANACOM 1/2 until an audio tone can be clearly heard.
This should occur between 55 and 65 on the tuning scale (fine tuning for the strongest
possible signal may be required).
On your dual trace oscilloscope, set both the inputs to AC.
Use Channel 1 of the oscilloscope to monitor the transmitted AM signal at tp3 of
ANACOM 1/1. The AM signal should be under-modulation, else ensure that the
BALANCE PRESET in the BALANCED MODULATOR & BANDPASS FILTER
CIRCUIT 1 is turned to fully clockwise.
Use Channel 1 of the oscilloscope to monitor the message signal at tp1 of ANACOM
1/1. On the oscilloscope, select Channel 1 as the triggering source.
On the ANACOM 1/2, the received signal is passed through a series of circuits
including the RF amplifier, mixer, IF amplifiers, diode detector and AF amplifier.
The demodulated signal can be observed at tp39 of ANACOM 1/2. Use Channel 2 of
the oscilloscope to monitor the demodulated signal at tp39.
Compare the received signal with the transmitted signal.
 Observe if there is any phase difference between the two signals.
 Examine if there is any distortion on the received signal as compared to the
transmitted signal.
Repeat the step (x) above by tuning the BALANCE PRESET in the BALANCED
MODULATOR & BANDPASS FILTER CIRCUIT 1 slowly counter clockwise.
Analyze the changes to the received signal. Record your observation for the cases of
100% modulation and over-modulation.
YL Foo, 2014 Feb
14
FACULTY OF ENGINEERING
LAB SHEET
ETN2016
ANALOG COMMUNICATIONS
TRIMESTER 2 (2013/2014)
Experiment 2:
TM202 – Frequency Modulation
Note: Students are advised to read through this lab sheet before doing experiment. On-thespot evaluation will be carried out during or at the end of the experiment. Your performance,
teamwork effort, and learning attitude will count towards the marks.
YL Foo, 2014 Feb
15
1. Objectives
Experiment Part 1: Frequency Modulation (FM)
 To examine a message signal, a carrier signal and a FM modulated waveform in time
domain.
 To examine a message signal, a carrier signal and a FM modulated waveform in
frequency domain.
Experiment Part 2: FM Demodulation
 To demonstrate the demodulation of FM signal.
2. Introduction
a) Frequency Modulation
An alternative system to Amplitude Modulation is Frequency Modulation (FM). In the
modulation scheme the frequency of the carrier signal changes linearly with respect to the
change in the amplitude of the information/message signal. The larger the amplitude of the
information signal, the further the frequency of the carrier signal is changed from its
starting value. The amplitude of the information signal determines how many times in a
second the change in the frequency of the carrier occurs. Notice in Figure 1 that the
modulation process does not affect the amplitude of the carrier.
Figure 1: Frequency Modulation
The general angle-modulated signal is given by
xt   Ac cosi t   Ac cos2f c t   t 
In FM, the amplitude of the modulated carrier is held constant and the time derivative of
the phase i(t) of the carrier is varied linearly with the information/message signal m(t).
The instantaneous frequency fi(t), having units of hertz (Hz), of x(t) is given by
1 di (t )
f i (t ) 
 f  1 d
dt
c 2 dt
2
Frequency modulation implies that ddt is proportional to the modulating signal. This yield
YL Foo, 2014 Feb
16
d  2 k m(t )
f
dt
Thus, in FM the instantaneous frequency varies linearly with the message signal.
fi  f c  k f m(t )
The term kf, expressed in Hertz per unit of m(t) (Hz/V), represents the frequency sensitivity
of the FM signal.
b) Frequency deviation
Consider a sinusoidal modulating information signal given by
m(t )  Am cos(2 f mt )
The instantaneous frequency of the resulting FM signal equals
fi (t )  f c  k f Am cos(2 f mt )
The maximum change in instantaneous frequency fi from the carrier frequency fc, is known
as frequency deviation f. Thus,
f  k f Am
The frequency deviation is a useful parameter for determining the bandwidth of the FMsignals. As an example, an information signal of peak voltage of 3 volt and a frequency of
10 kHz with a frequency sensitivity of 15 kHz/V would cause a FM carrier to change by a
maximum deviation of 45 kHz (i.e. 45 kHz above and below the original carrier
frequency). The carrier frequency would be swept over this range 10,000 times a second.
c) FM Modulator: The Reactance Modulator
Figure 2 shows the reactance modulator circuit.
The operation of the reactance modulator
i) The oscillator and the tuned circuit provide an unmodulated carrier frequency at the
collector of the transistor.
ii) The capacitor "Cf" and the resistor "Rbase" are used for phase shifting, and together
with the transistor, form a voltage-controlled capacitor. This voltage-controlled
capacitor is actually in parallel with the tuned circuit.
iii) The changing information signal being applied to the base has the same effect as
changing the bias voltage applied to the transistor and, this would have the effect of
increasing and decreasing the capacitance value of the voltage-controlled capacitor.
iv) As the capacitance is effectively in parallel with the tuned circuit, the variation in its
value will cause the resonance frequency to change and hence the carrier frequency
will be varied in accordance with the variation in the amplitude of the information
signal.
YL Foo, 2014 Feb
17
Figure 2: The Reactance Modulator Circuit
d) FM Demodulator: The Quadrature Detector
A FM demodulator extracts information from the frequency of a modulated signal. In
brief, a FM demodulator must be able to convert frequency variations into voltage
variations, with minimum amount of distortion. To achieve this, a FM demodulator should
have a linear voltage versus frequency characteristic. A FM demodulator can also be
called a discriminator. Figure 3 shows the block diagram of a quadrature detector which is
used as FM demodulator.
Figure 3: Quadrature Detector
The incoming signal is passed through a phase-shifting circuit. The degree of phase shift is
determined by the instantaneous frequency of the input signal.
The rules are:
i) If the carrier is unmodulated, the phase shift is 90.
ii) If the carrier increases in frequency, the phase shift increases beyond 90.
iii) If the carrier decreases in frequency, the phase shift decreases below 90.
YL Foo, 2014 Feb
18
The phase comparator circuit is used to detect any discrepancy in the phase of the two
input signals. It compares the phase of the original input FM signal with the output of the
phase shifting circuit. It then produces a voltage level, which depends on the result of the
comparison according to the following rules:
i) Phase difference of 90 will result in no change of the output voltage level.
ii) Phase difference over 90 will result in an increase of the output voltage level.
iii) Phase difference less than 90 will result in a decrease of the output voltage level.
As the phase changes, the voltage level moves up and down and reconstructs the
information/audio/message signal. A low pass filter (LPF) is included to reduce any highfrequency ripple. Consequently, the signal at the output of LPF closely resembles the
original information signal.
3. Apparatus
i)
ii)
iii)
iv)
v)
vi)
ANACOM 2
Power Supply: +12V at 1A, -12V at 1A (LJ PS2 IC Power 60 or PS4 System Power
90)
Set of 4mm patching leads
Dual Trace Oscilloscope
Function Generator
Spectrum Analyzer
4. Precautions
i)
ii)
The negative terminal of the oscilloscope probe and function generator must be
connected to the 0V reference on the circuit board for all measurements.
The Oscilloscope probe must be set to x1 in all measurements.
5. References
i)
ii)
iii)
Technical System, “An Introduction to Analog Communications – Curriculum
Manual AT02”.
LJ Technical System, “An Introduction to Analog Communications – Student
Workbook AT02”.
Taub and Schilling, “Principles of Communication Systems”, McGraw-Hill, 1986,
Chapter 3.
YL Foo, 2014 Feb
19
YL Foo, 2014 Feb
21
6. Experiment Procedure
6.1 Experiment Part 1: Frequency Modulation (FM)
6.1.1 FM Signal, Frequency Deviation, Frequency Sensitivity
i)
Connect the ANACOM 2 board to the power supply as shown below:
Figure 4: Power supply connections
ii)
Power on the Function Generator and set it up as follows:
 Frequency, fm = 10 kHz.
 Sine wave
 Make sure the Amplitude is not at its minimum level (not fully counterclockwise)
ANACOM 2 board setup:
 There are two frequency modulators to choose from - REACTANCE modulator
and VARACTOR modulator. In your report, state the one you have chosen to use.
 Set the CARRIER FREQUENCY preset in the modulator block to its fully
clockwise position.
 Set the GAIN preset of the MIXER/AMPLIFIER block to fully clockwise
position (MAX).
 Switch on the power supply.
iii)
iv)
Connect the oscilloscope Channel 1 probe to tp34 (test point 34), and trigger on
Channel 1. Select an appropriate time scale. On the oscilloscope you will see the
carrier signal, a high frequency (approximately 455 kHz) continuous wave generated
by the modulator block, which will be frequency modulated by a message signal.
Examine the carrier signal in time domain, and determine its frequency (fc) and peak
voltage.
Use a spectrum analyzer to examine the carrier signal in frequency domain. Connect
an oscilloscope probe to the spectrum analyzer, and then probe at tp34. At the
spectrum analyzer, adjust the frequency, span, and reference level accordingly until
you see a peak occupying a large part of the screen. A good starting point is to set the
center frequency at 455 kHz, span at 100 kHz, and amplitude – reference level at 20
v)
vi)
vii)
viii)
ix)
x)
xi)
xii)
xiii)
xiv)
dBm. Record the frequency of the peak. Is it the same as what you have calculated in
step (iii)?
Use a function generator cable (not an oscilloscope probe!) to connect the function
generator’s output to the AUDIO INPUT socket of the MODULATOR CIRCUIT
block.
Connect the oscilloscope Channel 1 probe to tp6 (test point 6), and trigger on Channel
1. Select an appropriate time scale. This is the message signal, a low frequency signal
generated by the function generator, which is used to frequency modulate a carrier
signal. Examine the message signal, and determine its frequency and peak voltage. Is
the frequency similar to that has been set on the function generator?
Use a spectrum analyzer to examine the message signal in frequency domain.
Connect an oscilloscope probe to the spectrum analyzer, and then probe at tp6. At the
spectrum analyzer, adjust the frequency, span, and reference level accordingly until
you see a peak occupying a large part of the screen. A good starting point is to set the
center frequency at 10 kHz, span at 20 kHz, and amplitude – reference level at 20
dBm. Record the frequency of the peak. Is the frequency similar to that has been set
on the function generator?
Set the amplitude of the function generator’s output, Am = 0V, by turning the
Amplitude knob fully counter-clockwise.
Use the spectrum analyzer to probe tp34. Set the center frequency at 455 kHz, span at
100 kHz. What do you observe? Is there any modulation?
Increase Am until the first set of significant sidebands appear. Is there any
modulation? Determine the frequencies of the sidebands. How are they related to fm
and fc?
What is the bandwidth occupied by this FM signal?
Increase Am until the second set of significant sidebands appear. What are their
frequencies?
Increase Am some more until the third set of significant sidebands appear. What are
their frequencies?
Increase Am further until the maximum. Enlarge the span to 200 kHz if necessary.
What happen to the sidebands? How many sets of significant sidebands do you
observe now? What happen to the carrier’s magnitude now? Estimate the bandwidth
occupied by this wideband FM signal.
6.2 Experiment Part 2: Frequency Demodulation
i)
ANACOM 2 board setup:
 There are five demodulator/detector circuits, i.e. DETUNED RESONANT
CIRCUIT, QUADRATURE DETECTOR, FOSTER-SEELEY DETECTOR,
RATIO DETECTOR, and PHASE-LOCKED LOOP (PLL) DETECTOR. Repeat
the following steps for each one of them.
 Link the OUTPUT socket of the MIXER/AMPLIFIER block to a detector’s
INPUT.
 Link the OUTPUT socket of the DETECTOR block to the INPUT of the LOW
PASS FILTER/AMPLIFIER block.
 Turn on the power supply.
YL Foo, 2014 Feb
23
ii)
iii)
Using an oscilloscope, monitor the modulating signal, i.e. the input to the
MODULATOR, and the demodulated signal, i.e. the output of the LOW-PASS
FILTER/AMPLIFIER block.
Adjust the GAIN preset (in the LOW PASS FILTER/AMPLIFIER block and/or
MIXER/AMPLIFIER block) until the amplitudes of the monitored waveforms are the
same. Note: If the amplitude does not match when the gain is already maximum, try
to get the display as clear as possible for the output signal. Compare between the two
signals. Is there any difference? Discuss. Explain whether demodulation of FM signal
has been performed.
YL Foo, 2014 Feb
24